Trade Journals for the Concrete Industry

Mechanical compaction of conventional concrete with poker vibrators is usually divided into two stages [1]. In the first stage (lasting 3 to 5 sec.), known as slumping, tiny air bubbles merge into larger ones, which typically remain in the lower layers of the concrete mixture. In the second stage (which lasts from 7 to 15 sec.), which removes entrapped air, air bubbles move upwards to the concrete/ formwork interface and escape. Release oils of different chemical compositions prevent bonding between the hardened concrete and the formwork. Water-based release agents were found to create fewer surface voids on the formed concrete surface than oil-based agents. Release agents must be applied in a thin film on the surface of the formwork. Excess amounts of release agent can result in poor quality of the formed concrete surface [2] and must be removed using a dry cloth or rubber scraper.